I just had 7 Supermutant Overlords attack, plus a glowing houd, is this normal? I'm level 49 I could barely survive and I was only in combat armour, was this a glitch or normal?

Yes, it gets worse the higher your level.

later on it will be Warlords and other tougher foes.

Like this pic taken at Warwick farm

http://ocau.com/pix/yxmcr

Interesting, I had no companion and them all coming at me, crazy fight, I used the ship wreck as cover

Just wait until nearly all of them have missile launchers and miniguns.

Yep, the first time it happened to me it came as a bit of a shock.

I had to head away from the settlement, backpedalling.

It made me improve the defences there and put up a wall to slow them down.

I'm up to level 70 and am beginning to see missile-launching enemies. So I'm starting to build missile turrets as well. You counter their attack with the same attack. One way to defend your settlement is to have a large population (20+), which can parlay into great firepower. Turrets only have 20-30 damage resistance and can be knocked down in seconds. But if you give settlers better armor, they last longer and can get back up while knocked down. And for the turrets that need power, don't "daisy-chain" them, because if one gets damaged, the whole chain loses power. Hook them all to power pylons or conduits, which can't be damaged.

I trained myself to do this in addition to the defense tips already given:

Every time I fast travel to a settlement under attack, the very first thing I do upon arriving is to immediately equip my flare gun and shoot a flare to try and get Minuteman help. When it works, the extra 5 or 6 defenders (I'm on Survival mode- dunno' if that # scales with difficulty) really help if the attacking force is tough. Worst case, they're free bullet sponges who get tore up instead of you and your settlers.

Wait 'til you get Gunners in PA along with just the normal guys having heavy weapons.